1. Field of the Invention
The present invention relates to an image processing apparatus that performs an image process on image data for printing. More particularly, the present invention relates to an image processing apparatus that performs a high quality color conversion process in printing a document constituted of halftone areas.
2. Description of the Related Art
In the field of image forming apparatuses, such as copying-machines or printers, that adopt an electrophotographic process or an inkjet printing system, multi-color digital copying-machines and multi-function printers that can reproduce color images with high image quality have been commercialized, due to advancements in digital image processing technology.
Such an image forming apparatus copies document images containing text, line drawings, pictures, or combinations thereof. In order to desirably reproduce such document images, it is necessary to perform an appropriate image process on each type of document image.
Under these circumstances, various operation modes, such as a text mode, a text/picture mode, and a picture mode, are available in the image forming apparatuses. One of the operation modes is selected based upon a document type, and then an image process based upon the selected operation mode is carried out.
It is, however, extremely bothersome for a user to switch the operation modes for each document. Moreover, if the user selects an inappropriate mode, then an inappropriate image process is carried out to the document. This often leads to image deterioration, resulting in wasted copy.
As a way to solve the above problems, Japanese Unexamined Patent Publication No. 251406/1996 (Tokukaihei 8-251406 (published on Sep. 17, 1996)) and Japanese Unexamined Patent Publication No. 36907/2000 (published on Feb. 2, 2000) teach automatically discriminating a document type in the image forming apparatus, so as to use different image processes depending on the result of discrimination.
Japanese Unexamined Patent Publication No. 251406/1996 teaches (i) segmenting supplied image data into a line-only area, a halftone-only area, a continuous-tone-image-only area, or a combination thereof, and (ii) switching g a filter process, a color correction process, and a tone process depending on the result of segmentation.
Further, Japanese Unexamined Patent Publication No. 36907/2000 teaches calculating a gloss level of a document to select an appropriate image mode (text/picture mode or picture mode) based upon the gloss level (color correction process and γ correction process are switched based upon the selected image mode).
However, Japanese Unexamined Patent Publication No. 251406/1996 has a problem that an appropriate image process would not be carried out to, especially, an image containing a halftone area. The following specifically describes the problem.
Usually, an image forming apparatus adopting the electrophotographic system or the inkjet printing system performs a color conversion process so as to convert supplied image data of a first color space (i.e., RGB (red, green, blue)) into data of a second color space (i.e., CMY (cyan, magenta, yellow) or CMYK (cyan, magenta, yellow, black)) that is more suitable as an output of an image forming section.
Generally, the image forming apparatus has a narrower gamut than a printed-picture (an image constituted of halftone areas) or a photographic-picture (an image constituted of continuous-tone areas). Therefore, in printing the printed-picture or the photographic-picture by the image forming apparatus, if the color correction process is performed by placing importance on the tone, then images are reproduced with paler colors than original colors. On the other hand, if importance is placed on the density or the color saturation, tones tend to be reproduced inaccurately. Especially in printing a document containing a high frequency halftone (document with high image quality, such as a book of fine art), the image quality deteriorates.
Further, in printing a document containing a low frequency halftone (e.g., newspapers), an attempt to accurately reproduce pale colors of halftone areas (e.g., pictures) in the document causes the following problem. Specifically, if the document itself is colored (e.g., newspapers), the color of the page-background is also reproduced. This causes poor reproduction of text.
Thus, in processing an image containing a halftone area, different factors need to be taken into consideration depending upon whether the document is a high frequency halftone document or a low frequency halftone document. It is therefore preferable to perform an image process that is more segmented. The method of Japanese Unexamined Patent Publication No. 251406/1996, however, does not allow a halftone area to be further segmented for determination. Accordingly, one type of process is performed on all halftone areas. This may cause deterioration of image quality depending on the document type.
Further, Japanese Unexamined Patent Publication No. 36907/2000 teaches measuring a gloss level of the document when the image mode is either the text/picture mode or the picture mode. This enables one to perform more segmented discrimination of the pictures (halftone areas). However, in order to discriminate by detecting a gloss level of the document, it is required to include a sensor for detecting a specular reflection component from the document, and a member for calculating the gloss level.